The present invention relates to a method of producing metal parts having magnetic and non-magnetic portions. Such parts are widely used in various machines and instruments. These parts are conventionally produced by joining metals having different or contrasting properties, e.g. magnetic and non-magnetic properties, high and low electric resistivities, different coefficients of thermal expansion, different Curie points, and dissimilar strength and ductility characteristics. The metals having the above-mentioned contrasting properties are usually joined together by welding, brazing, rivetting, gluing, pouring, cladding or plating as well as mechanical coupling by hot and cold pressure processing techniques.
However, when metal parts are produced by these conventional methods, the properties of the joined metals forming the magnetic and non-magnetic portions of particular parts are deteriorated. Besides, the production of parts composed of several separate pieces causes a number of technological problems. For example, in the prior art practice metals having different crystalline structures, such as a non-magnetic steel of the austenite type and a magnetic steel of the martensite type have been usually joined by welding techniques. To avoid the formation of hot cracks, the austenite steels have been welded while employing small linear amounts of arc energy in combination with maximum possible cooling rates. On the contrary higher linear amounts of arc energy accompanied by tempering or heating have been used to avoid the formation of cold cracks during welding of the martensite steels. However, if the same conditions are employed for welding both the magnetic and non-magnetic metals, the properties of one of the metals being joined are degraded resulting in poor strength, ductility and impact toughness. In addition, during welding, the metals being joined are heated to temperatures which are very close to their melting points and their initial structures change and uncontrolled magnetic properties originate at the transition area of the part produced. The local mixing of metals also contributes to the generation of non-controlled properties within the seam. The strength of the welding joint is, as a rule, lower than that of the base metal. In some cases welding is improper, or it may be extremely difficult if at all feasible to weld some very dissimilar metals.
Joining metals by hot or cold pressure processing or by pouring one metal onto another provides satisfactory durability of the part produced. However, such a one-piece or integral part, which is in fact a bimetal, contains metals differing in their crystalline structures, e.g. an austenite non-magnetic structure of one metal and a martensite magnetic structure of the other metal. Heat treatment of a workpiece of this bimetal will cause a heavy deterioration of the magnetic and the mechanical properties of one of the metals constituting the bimetal part, and therefore a heat treatment can hardly be used, or in some cases is completely improper for improving the properties of its magnetic and non-magnetic portions.
Mechanical coupling and gluing techniques of magnetic and non-magnetic metals cannot provide for a high durablility, long-time strength and operability of the parts produced.
It is common knowledge that the magnetic properties of a part made of a magnetic material weaken when it is heated, but attempts to use a localized heat treatment in the case of a stainless steel magnetic workpiece brought no favorable results, since the part produced by this method had portions with weakly pronounced magnetic properties. Completely non-magnetic portions can not be obtained. Thus, parts produced by this technique were found unsuitable for practical use.
Accordingly, the main problem resides in choosing a proper metal for making a workpiece and determining the mode or conditions of its heat treatment.